Materials requirements for aerospace applications are going through sweeping changes primarily due to defense related goals which include higher thrust-to-weight ratios, faster cruising speeds, increased altitudes and improved flight performance. When these goals are translated into material requirements, the general theme of incorporating lightweight material possessing increased strength at higher operating temperatures emerges. Ceramic materials such as silicon nitride (Si.sub.3 N.sub.4), silicon carbide (SiC), silicon aluminum oxynitride (SiAlON), alumina (Al.sub.2 O.sub.3) and titanium diboride are some of ceramic materials that have been found useful for high temperature applications.
However, the replacement of metal parts with ceramics is not as simple as has been initially suggested. One area which has been especially troublesome is the use of ceramic replacement parts in mechanical systems that require extreme pressure (EP) lubrication at elevated temperatures. Examples of mechanical systems can be found in turbojet engines where sliding contact occurs between parts sliding and rotating in bearings. These include subassemblies such as turbine shafts wherein the shafts rotate inside journal bearings or rolling contact bearings, valves wherein valve bodies slide against mating surfaces in their opening and closing motions, and afterburner gates which turn on cylindrical bearings sliding around shafts or on shafts rotating inside cylindrical bearings.
The problems that arise in the use of ceramic replacement parts in mechanical systems that require extreme pressure (EP) lubrication at elevated temperatures are due primarily to two conditions. The first is that the ceramics are being used under higher temperatures conditions than the metal that it is replacing. Lubricants that work well with metals at relatively low temperatures (below 1000.degree. F.) do not necessarily function as lubricants above 1000.degree. or 1200.degree. F. Lubricants that have been used at these relatively lower temperatures have been found to polymerize, oxidize and/or thermally degrade into a solid at the higher temperature conditions that they are now being tested under. In addition, the solid material that the lubricants thermally degrade into, have in some instances been found to be a hard sticky substance that increases the coefficient of friction between the surfaces that it is to lubricate and could cause a seizure of the moving parts. Also, this solid material has been found to be extremely difficult to remove from surfaces that it has contacted. This translates into increased downtime in the event that the maximum temperature, for which the lubricant has been designed, has been exceeded.
The second problem that arises in the use of ceramic replacement parts in mechanical systems that require extreme pressure (EP) lubrication at elevated temperatures, is that the lubricants that lower the coefficient of friction between metals at elevated surfaces do not necessarily function as lubricants for ceramic surfaces. The use of the wrong lubricant will increase the coefficient of friction between the ceramic surfaces and may cause a greater tendency for the surfaces to seize than when no lubricant is used.
U.S. Pat. No. 3,978,908, issued to Klaus et al, discloses a method of die casting metal. In this method the mold or die surface is contacted with a vapor lubricant or parting agent such as an alkyl phosphate or an aryl phosphate, e.g., tricresyl phosphate and tributyl phosphate. The vapor lubricant may be applied at temperatures as high as 1200.degree. F.
A great deal of work has been done on the vapor lubricants of U.S. Pat. No. 3,978,908. Klaus et al in an article entitled "Structure of Films Formed During the Deposition of Lubrication Molecules on Iron and Silicon Carbide", presented as a Society of Tribologists and Lubrication Engineers paper at the ASME/STLE Tribology Conference, October 1988, have found that impinging tricresyl phosphate (TCP) molecules thermally decompose and interact with the iron surface to form two types of crystalline structures. One structure apparently consists of large, oriented cementite (Fe.sub.3 C) crystals. Klaus theorized that this layer probably grows by diffusion of carbon fragment from the TCP into the original foil material and subsequent reaction. The results suggest that iron in some form acts as a catalyst for the initial adsorption and decomposition of the TCP, and there is some iron transport process that can operate through several thousand monolayers of coating. With regard to the SiC substrate, there was no evidence to suggest that the ceramic played a role other than to provide thermal energy.
Alkyl and aryl phosphates require the presence of a metal oxides, such as iron in the form such as Fe.sub.2 O.sub.3, which react with the phosphate at high temperature to form the lubricant. Other metal oxides such as oxides of other transitional metals such as nickel, chrome or manganese, for example, have also been found to react with the phosphate at high temperatures to form lubricants. These other metal oxides are also obtained from the metal surfaces which were being lubricated.
When lubricating ceramic surfaces of high temperature ceramics, such as silicon nitride (Si.sub.3 N.sub.4), silicon carbide (SiC), silicon aluminum oxynitride (SiAlON), alumina (Al.sub.2 O.sub.3) and titanium diboride, there are no metal oxides present to react with the phosphate. If Fe.sub.2 O.sub.3 is added to the parting agents so that the phosphate containing parting agent can act a lubricant at high temperatures to lubricate ceramic surfaces, the Fe.sub.2 O.sub.3 will also react with silica in the ceramic to produce an FeSi phase which will reduce the strength of the ceramic and cause its surface to degrade. The formation of an FeSi phase and its detrimental effects on ceramics is well known. Those skilled in the art would not normally think of using iron in a parting agent that is to be used for high temperature ceramics.
It would be advantageous, therefore, to provide a lubricant that can be used for lubricating ceramic surfaces, such as silicon nitride (Si.sub.3 N.sub.4), silicon carbide (SiC), silicon aluminum oxynitride (SiAlON), alumina (Al.sub.2 O.sub.3), aluminum phosphate (AlP), zirconia (ZrO.sub.2) and titanium diboride at temperatures above 1000.degree. F.
The principal object of the present invention is to provide a system for lubricating ceramic surfaces at elevated temperatures which does not require the formation of transition metal silicides to form a lubricant.
Another object of the present invention is to provide a lubricant for use with Si.sub.3 N.sub.4 that can be used at temperatures above 1000.degree. F.
Additional objects and advantages of the present invention will be more fully understood and appreciated with reference to the following description.